Speeding Bullet

Astronomers often use the amount of X-ray emission from a cluster of galaxies as a surrogate of the mass of the cluster. This is because the hot, X-ray emitting gas has an enormous outward pressure which is opposed by the gravitational field of the matter in the cluster. This balance can be upset if the hot gas is re-heated, making such mass determinations suspect. An example of cluster heating has been revealed by a deep X-ray image of the cluster Abell 3376, shown above, taken with the XMM-Newton X-ray observatory. This image shows a bullet shaped region of emission in the cluster. Astronomers believe this emission is produced by smaller groups of galaxies merging with the main cluster. Radio observations of the cluster show shocked gas ringing the cluster, probably ripples produced by the infall of these subcluster "bullets".